1. Field of the Invention
The present invention relates to a bearing test method and a bearing test device for bearings in which a non-contact state is maintained between a shaft element and a bearing element in a normal rotation state. In addition, the present invention relates to a motor bearing monitoring device that can detect an abnormality in a motor or that a motor's life is near its end, where the motor utilizes the bearing described above. Furthermore, the present invention relates to a storage device such as a hard disk drive equipped with the motor monitoring device.
2. Description of Related Art
Bearings in which a shaft element and a bearing element retain a non-contact state between them in a normal rotation state have been in use for some time. One such bearing, for example, is a dynamic pressure bearing.
A dynamic pressure bearing has a structure in which grooves for dynamic pressure generation through oil, air or other fluids are formed between the shaft element and the bearing element; when the shaft element and the bearing element rotate relatively, the dynamic pressure increases as the rotation speed increases, and when the rotation speed exceeds a predetermined number of revolutions (number of float revolutions) a fluid film causes the shaft element to float up from the bearing element and shift to a non-contact rotation state. Conversely, when the rotation speed falls below the predetermined number of revolutions (number of contact revolutions), the state switches back to the contact rotation state. Such a dynamic pressure bearing is used as a bearing for a high-speed rotation motor, such as a hard disk drive motor.
A higher number of float revolutions or of contact revolutions translates into a longer contact rotation state when the motor starts or stops. The longer contact rotation state device increased amount of wear on the bearing section during intermittent motor drive, which shortens the life of the bearing. Additionally, due to the fact that lubricating oil's viscosity reduces inversely to the rise in the oil's temperature, the number of float revolutions or contact revolutions increases the higher the temperature; consequently, there is a risk that the amount of wear on the bearing section would worsen significantly. Thus, the number of float revolutions or of contact revolutions must be maintained at a sufficiently low level against the number of rated revolutions. As a result, the number of float revolutions or contact revolutions of a dynamic pressure bearing is considered to be one of the critical testing items of its properties.
In the past, the measurement of the number of contact revolutions of dynamic pressure bearings was generally done using the AE (Acoustic Emission) method. The AE method is a method in which the acoustic energy generated by metals coming into contact with each other is detected by a vibration sensor and converted into voltage.
FIG. 23 shows an example of an AE waveform based on the AE method obtained from a motor equipped with an oil dynamic pressure bearing, along with changes in the number of motor revolutions. The strength of vibration detected by an AE sensor from a motor bearing section is converted into voltage, and the result is observed as an AE waveform on an oscilloscope screen. The number of motor revolutions is a measured value of the number of revolutions provided by a tachometer (voltage value when using a tachometer).
A method that uses the AE method to detect the number of float or contact revolutions of the dynamic pressure bearing or abnormalities in general bearings entails the following problems.
First, in a dynamic pressure bearing such as an oil dynamic pressure bearing, the two metal members (the shaft element and the bearing element) are lubricated with lubricating oil between them, which causes the contact sound made when the two metal members come in contact to be extremely small, and it is difficult to completely eliminate noise from detection signals. Consequently, the number of contact revolutions cannot be detected with high precision.
Additionally with the AE method, a sensor for detecting contact sound must be in direct contact with the bearing section or a motor composition part in which the bearing is assembled; when the sensor contact is imperfect, the detection precision for the contact sound is decreased, and pass/fail judgments on bearings cannot be adequately made.
Furthermore, due to large deviations in detection sensitivity, making pass/fail judgments on bearings based upon detection signal waveform is difficult. For instance, when the bearing switches from a non-contact rotation state to a contact rotation state, metal contact sound is detected and the detection signal level rises, but because there are large deviations in the detection signal level it is difficult to establish a threshold voltage that would be the criterion to determine that the bearing has switched to a contact rotation. As a result, automatic testing cannot be done and there is no choice but to rely on human sensibility. This leads to ambiguous pass/fail judgments.
A memory device, such as a hard disk drive, is made up of a disk driving motor, on which a plurality of hard disks is mounted, and a magnetic head to record on or retrieve from the hard disks; the disk driving motor's rotor is supported in a freely rotatable manner against a stator by a ball bearing or a dynamic pressure bearing.
In such a hard disk drive, if there is an abnormality with the disk driving motor or the disk driving motor reaches the end of its life, operation errors occur in retrieving and recording. In extreme cases, uneven rotation or a motor lock state occurs and recording and retrieving information become impossible.
Operation errors with disk driving motors are frequently caused by abnormalities with the bearing or due to the fact that the bearing is reaching the end of its life. For example, with a ball bearing, worn balls, depletion of lubricant, or contamination by foreign matters such as metal pieces can cause rotation abnormalities in the motor. Also, the following can cause rotation abnormalities in a motor when an oil dynamic pressure bearing is used in the motor: contamination by foreign matter such as metal pieces in the gap between a shaft element and a bearing element that is rotatable in a non-contact manner via a lubrication film; depletion of oil in the gap between the shaft element and the bearing element; or wear on the bearing element. Similarly, when an air dynamic pressure bearing is used, contamination by foreign matter in the gap between the shaft element and the bearing element or wear on the bearing element can cause rotation abnormalities in the motor.
However, conventional memory devices such as hard disk drives are not equipped with any means to predict in advance occurrences of operation errors caused by these abnormalities in the motor bearing. Consequently, the operator first becomes aware of operation errors only when fatal problems such as uneven motor rotation or motor lock occur. But in these situations, there is a high possibility that a critical problem of not being able to read recorded data has already occurred.
In the meantime, a method to monitor the rise in motor drive current is known as a method for detecting abnormalities in the motor. However, an abnormal rise in motor current occurs after a burn caused by metal friction contact of motor elements of the motor bearing, so that by the time a rise in the motor current is detected it is often already too close to a state in which recorded data are unretrievable. Consequently, it is impossible with this method to detect abnormalities in a motor bearing while recorded data are still transferable.
As a result, in conventional memory devices, an adverse effect of losing recorded data when the recorded data become unretrievable due to motor operation error caused by a motor bearing abnormality is considered unavoidable to some extent.
In view of the problems found in conventional bearing test methods, the present invention suggests a bearing test method and a bearing test device that can detect the rotation state of a bearing without using the AE method. In particular, the present invention suggests a bearing test method and a bearing test device suitable for testing dynamic pressure bearings assembled in hard disk drive motors.
Additionally, the present invention suggests a motor bearing monitoring device that can detect abnormalities in bearings of motors mounted on memory devices before an operational shutdown.
Furthermore, the present invention suggests a memory device that is equipped with the motor bearing monitoring device and that can warn beforehand that recorded data would become unretrievable or that can backup the recorded data before the recorded data become unretrievable.